We have analyzed the effect of expiratory flow limitation on the pattern of lung emptying in normals. We have evidence indicating that airway closure is not required to produce a Phase IV rise in expired gas concentration. We have quantified the effects of CO2 breathing on lung mechanics in normal subjects and find significant increases in total lung capacity and decreases in lung conductance. Thoraco-abdominal mechanics at rest in sitting and supine normal subjects and patients with obstructive lung disease have been quantified. All patients showed varying degrees of deviation from the normal pattern.